FIELD OF THE INVENTION
This invention relates to electronic hardware component structural design and more particularly to headers for attaching component modules to a mother board.
Electronic components are typically manufactured in the form of electronic subassemblies called modules which are later electrically and mechanically connected to a larger circuitboard or mother board for use in electronic systems. Many of these modules require interconnect devices referred to as header assemblies in order to connect the modules to the mother board. These interconnect devices are used today in the electronic industry for such applications as SIP (single inline packages) and ZIPS (zigzag inline packages). Headers are supplied by companies such as NAS and others.
FIG. 1 shows a perspective view of the relevant structures described above. A module 1 in this case is a small circuit board having a plurality of electronic devices attached to it. The devices can be packaged integrated circuit chips 2 and/or discrete devices 3 such as individual resistors, transistors and diodes. Currently, many integrated circuits are themselves packaged as modules which may connect directly to the header. These components will also benefit from improved header design.
Each module has a series of conductive contact pads 4 which form the electrical (and usually mechanical) connection points between the module and the rest of the system. The contact pads are typically soldered to pins 5 extending from one side of a header body or strip 6. The pins also extend from the bottom side of the header strip and are positioned to insert through holes 7 in the mother board 8. During the reflow process, the pins are soldered to the mother board.
Certain problems occur using the currently available header designs. During reflow, molten solder necessarily contacts the lower end of the pins to electrically connect them to the mother board. Heat from this molten solder travels up the pin to heat any structures which are connected to the pin. This includes the header strip, the solder connection between the top of the pin and the module pads and structures further within the module. If it is hot enough, some plastic or solder structures may melt. This can cause corruptions of header and module structures resulting in device failures.
Corruptions can include desoldering, where molten solder flows or is jarred away from its proper position. This problem is most frequently observed at the pin/module pad solder connection points. During reflow, either gravity or other mechanical acceleration caused by shock or vibration dislodges enough molten solder from the pad region to cause an open circuit. Sometimes, pad and pin are slightly biased away from each other such that when the solder becomes fluid, the pin may spring away from the pad creating an open circuit. The molten solder, as it changes position can also form shorts with adjacent areas.
Other corruptions can occur as cold solder joints. In this case, the remaining solder, when it cools, may not be strong enough to maintain an adequately conductive connection. When the completed system is subjected to its operational environment, the electrical connection may fail.
Corruption is not restricted to the pad/pin solder connection. Heat may continue traveling along any thermally conductive path damaging other solder connections 9 existing on the module. Heat from reflow can even damage the module deices themselves.
Another possible corruption is the partial melting of the header strip plastic during reflow which allows the pin to shift position within the strip. When the strip cool and hardens, the pins may be positioned out of alignment. Depending on the header design, this form of corruption can be very widespread.
Generally, there are two types of header designs. FIG. 2 shows a cross-section of a Type-I header having a plastic body or strip 6 with a series of slats or holes through which pins 5 are inserted. The strip is usually made of a glass-filled thermal plastic or liquid crystal polymer and the conductive metal pins are usually cold-stamped from a leadframe and machine inserted. This type of header is very susceptible to reflow corruption because: 1) the pins may not rigidly held in position within the strip in the first place; and 2) a significant surface area of the pin is in intimate contact with the strip, allowing transmission of heat to the strip.
FIG. 3 shows a second type of header (Type-II) where the header strip 6 is made of an epoxy glass material which is much less susceptible to heat damage and the pins 11, 12 and 13 are made of varying shapes with features which provide more rigid positioning within strip holes 14, 15, 16 and 17. The first pin 11 has a swage 18 with a star-like cross-section. The lateral dimensions of the swage are slightly greater than the diameter of the hole 14 in the strip. Since the strip material is somewhat resilient, the hole slightly gives way as the pin is inserted, tightly gripping the pin.
Pins and holes having other shapes operate in the same manner. A pin 12 having a rectangular cross-section may be inserted in star shaped 15 or round 16 holes to provide a tighter fit. A pin 13 having a rounded cross-section and a tapered or widened swage 19 may be inserted into star shaped, round or square 17 type holes. The workable combinations of pin, swage and hole shapes are myriad.
Although the Type-II style headers have much more rigid pin header connections, they too suffer from some pin position shifting and transmitting heat to the module during reflow. Even spring contact type headers, as seen in FIG. 4, which do not form a solder connection between the pins and module can suffer from this problem. Type-II style headers may also be susceptible to pin position shifting because of the reduced surface area of the pin in contact with the strip allows any melting to result in a greater shift.
It would be advantageous, therefore, to have a header which prevents heat corruption of header, module and their interconnection during the reflow process.